A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. An LTE system also provides seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) communicating with a plurality of mobile stations, referred as user equipments (UEs).
An LTE system uses hybrid automatic repeat request (HARQ) at its Physical (PHY) layer to enhance data transmission quality while the HARQ procedure is controlled by Medium Access Control (MAC) or higher layers. HARQ is an error correction mechanism combining forward error control (FEC) and automatic repeat request (ARQ). At the transmitter side, error detection bits are added to the transmission data. The receiver decodes the received bits and sends an acknowledgement (ACK) or negative acknowledgement (NACK) back to the transmitter based on whether the transmitted data can be decoded correctly. The receiver sends the ACK or NACK by setting the corresponding HARQ bit(s) over a reverse control channel. In particular, in the LTE system, upon receiving downlink data from an eNB, a UE can send HARQ feedback information to the eNB via a Physical Uplink Control Channel (PUCCH). The current PUCCH supports up to 4 bits HARQ feedback information. The HARQ process improves the system through output. However, issues arise for the existing HARQ feedback channel design with enhancements to the LTE system.
Enhancements to the LTE system (LTE-Advance system) are considered so that it can meet or exceed International Mobile Telecommunications Advanced (IMT-Advanced) fourth generation (4G) standard. One of the key enhancements is to support bandwidth up to 100 MHz and be backwards compatible with existing wireless network systems. Carrier aggregation (CA) is introduced to improve the system throughput. With carrier aggregation, the LTE-Advance system can support peak data rate in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the component carriers.
In a mobile network, the bandwidth requirement of a UE changes with the amount of data the UE is transmitting and receiving. Carrier aggregation allows the mobile network to use the bandwidth more efficiently. In particular, carrier aggregation allows asymmetric number of downlink and uplink component carriers for each UE. For example, a UE with multiple CC capability can be configured to have five DL component carriers and only one UL component carrier in Frequency Division Duplex (FDD) system; or five DL portions and only one UL portion in Time Division Duplex (TDD) system. Due to the asymmetric UL and DL CC configuration, the payload size of the uplink HARQ increases significantly. For example, if five DL component carriers are configured, up to 12 bits are needed to carry the HARQ feedback information for FDD, and up to 47 bits are needed for TDD. The current non-CA PUCCH channel format, however, only supports up to 4 bits for HARQ feedback information.
Therefore, at least one new PUCCH channel format is needed for the uplink HARQ information. To be backward compatible, the LTE system needs to support both the non-CA-format uplink HARQ and the new CA-format uplink HARQ. Furthermore, the non-CA-format HARQ has better resource utilization efficiency, while the CA-format HARQ is less efficient. Depending on the application scenario, it is thus desirable that the applied HARQ feedback channel format changes accordingly to achieve better resource utilization efficiency. However, the wireless link is not reliable and control messages and data may be lost during transmission. This will result in information mismatch among UEs and eNBs. Blind decoding at the eNB side introduces higher computation complexity and performance degradation. To solve the problem, a HARQ format synchronization scheme between UEs and eNBs is required. A predefined rule for HARQ format switching in both UEs and eNBs is needed.
Another issue for HARQ feedback channel design in an LTE-Advance system is the physical resource allocation for the uplink HARQ. Due to the asymmetrical UL and DL carrier components for an UE, there may be only one HARQ feedback channel on a specific UL component carrier for multiple scheduled transport blocks (TBs) in more than one DL CCs. Therefore, the current non-CA based implicit resource allocation, which depends on the logical address of the downlink scheduler, cannot be used. Implicit resource allocation will create multiple candidate resource locations for the feedback due to multiple DL schedulers in the same scheduling period (e.g., subframe in LTE). Due to unreliable decoding results of the DL schedulers, an eNB does not know which resource location a UE will apply and thus has to reserve all candidate resource allocations. A solution is sought to allocate resource for HARQ feedback channel more efficiently for CA mode.